Automatic frequency controls (AFCs) are well known, and have been used in a wide variety of applications. In conventional applications, an AFC functions by adjusting a system variable control oscillator (VCO) based on the detected input signal frequency (e.g., by detecting the peak value following bandpass filtering). Even in applications using digital input signals it is common to rely partially upon analog circuitry to implement an AFC. Thus, one may typically find digital AFCs implemented with an analog VCO preceded by a DAC (digital to analog converter).
A problem with such conventional AFCs is their reliance on analog circuitry in the feedback loop. Where the AFC is used with digital signaling, the analog components add both size and cost to the AFC, and limits the full processing capabilities achieved by an all digital implementation. In other words, an all digital implementation will typically yield both higher accuracy and speed than one relying on analog circuitry.
One possible solution for an all digital implementation in the field of communications, described in N. Sollenberger, Low-Overhead Symbol Timing and Carrier Recovery for TDMA Portable Radio Systems, IEEE Transactions on Communications, October 1990. This proposed digital AFC provides for a one symbol delay from the output of a differential demodulator for received coherent symbols, and has a frequency offset estimator using differential phase error magnitudes from the differential constellation points accumulated on a per symbol basis, along with a symbol timing estimate, to output an offset estimate to feedforward first-order carrier recovery loop. However, a major drawback with this AFC is its requirement of a relatively high C/I (carrier to interference) ratio, as much as 17 to 25 dB in Rayliegh fading, and it relies on a solely data driven approach (i.e., it assumes no knowledge of the data being communicated). In many communications environments, such as digital cellular radiotelephony, requiring a 17 dB would lead to unacceptable frequency drift since the communications systems are designed to operate at much lower C/Is, sometimes as low as 3 dB.
There remains therefore a need for an improved digital AFC that overcomes these problems.